This invention relates generally to an electronic circuit of the type having two states and particularly relates to such a circuit capable of being activated by the passage of a minute electric current through the human body.
It has long been recognized that the size of electronic hand-held calculators is limited strictly by the size of the pushbuttons needed to actuate the electronic circuits. Since the size of the human digit cannot be reduced, neither can the size of the buttons which activate the circuit.
Hence, in accordance with the present invention, an electronic circuit is provided which has two states, at least one of which is a stable state. This circuit can be activated by the passage of an electric current through the human body, the current having a magnitude on the order of a few nanoamperes. Hence, such a circuit when held by the user can be activated by a conductive pin or the like, whereby the current flows between the circuit and the pin through the body of the wearer.
Switches which can be activated by the human touch have been proposed before. For example, certain elevator call buttons are of this type. When they are touched by a person the button is activated by the body capacitance to ground or earth. Other switch mechanisms are known which utilize the stray electromagnetic fields such as those generated by a 60 hz electric line.
On the other hand, the conventional mechanical switches which are presently utilized for controlling handheld electronic calculators have certain disadvantages. They contain moving parts, including springs, levers, knobs and depressable buttons. As a result, the switches have a finite lifetime and after a given number of operations the moving parts break or suffer excessive wear so they are no longer useable. As pointed out before, it is difficult and expensive to miniaturize mechanical switches. Besides, if they are too small, they can no longer be operated by a human operator.
Other mechanical switches are also known which offer certain advantages over the mechanical switch above described. This new type switch is called a conductor loaded elastomeric switch. However, the elastomeric switches have a characteristic of the key pressure versus contact resistance which can generate continuous noise low pressure key depressions. The certainty of conduction of an elastomeric switch is a function of the applied pressure. This is even more critical than for the conventional mechanical switches.
In contrast the electronic switch of the present invention can be made extremely small and requires no contact pressure beyond that needed to cause an electric current to flow. The current needed for activating this type of switch is on the order of 5 nanoamperes. A publication by Peter Strong entitled "Biophysical Measurements" published by Textronix, Inc., Oregon in 1970, states on pages 255 - 259 what the current threshold is for electric shock under certain conditions. For example, experiments have shown that up to 300 microamperes can be applied to the surface of the body such as from one arm to the other. Such currents are reasonably safe for most subjects. Much smaller currents can be tolerated where the current is applied internally to the body. In this case, the current may flow via the arterial system directly to the heart. In that case, a current of 15 microamperes is considered a safe upper limit. It should be noted in accordance with the present invention the current needed to activate the circuit is no more than a fraction of a microampere which is orders of magnitude below the safe upper limit of currents applied to the human body.
It is accordingly an object of the present invention to provide an electronic switch suitable for hand-held instruments such as calculators and the like which have no moving parts.
A further object of the present invention is to provide such an electronic circuit which can be readily activated by the operator by means of a conductive pin or the like.
Another object of the present invention is to provide a circuit of the type discussed which is inexpensive in manufacture and suitable for use with electronic instruments.
Still a further object of the present invention is to provide an electronic circuit which can be readily activated and which is particularly suitable for miniaturized applications.